1. Field of the Invention
The present invention relates firstly to a novel type of hydrogenation reaction vessel or "reactor". It also relates to hydrogenation methods implemented within such a reactor, and more generally to the use of such a reactor.
The present invention was developed in the context of hydrogenation reactions in the gaseous phase, generally implemented at high temperature and under high hydrogen pressure, in thick-walled reactors. Such reactors are to be found in particular in installations in the chemical and petrochemical industries. They are used in particular to constitute the cores of hydrocracking units in modern oil refineries.
Such reactors have their walls mainly constituted by a lightly alloyed ferritic steel (generally of the 2.25 Cr-1 Mo type) and they can be caused to work under conditions as severe as 170 bars of hydrogen pressure (17.10.sup.6 Pa), at 450.degree. C. Some of them have walls of a thickness of about 300 mm and they can weigh up to more than 1000 tonnes.
Such reactors must be designed so that under their conditions of use, they withstand both creep and attack from hydrogen. It is those two types of damaging phenomenon, which unfortunately are mutually synergistic, that limit the conditions under which such reactors can be used and which determine the nature of the material constituting the walls of such reactors and the thickness of said walls. It should also be observed at this point that the more or less corrosive nature of the reaction medium is also, of course, to be taken into account.
In the wall structure of the reactor, hydrogen attack gives rise both to surface decarburization of said walls and to the appearance of pockets of methane in the thickness of said walls; said methane being the result of chemical reaction between atomic hydrogen which has diffused through said walls and the carbon and/or carbides present in the steel constituting said walls. Within said methane pockets, pressure is extremely high and very high levels of stress are thus generated in the thickness of the walls of the reactor. Such stresses accentuate the harmful effects of creep and intergranular cracking then appears.
The person skilled in the art is very aware of the damage inherent to hydrogen attack due to said hydrogen diffusing through the metal walls of reactors. It is specified at this point that said diffusion is mainly due to atomic hydrogen. Molecular hydrogen (and indeed any other molecule) is too large to penetrate into the structure of the steel. To enable hydrogen to be absorbed into said structure it is therefore necessary for there to be a prior step of molecular hydrogen decomposing into atomic hydrogen, which atomic hydrogen is small enough to penetrate and diffuse within the steel. Such decomposition of molecular hydrogen into atomic hydrogen naturally depends on the temperature and pressure conditions at which said molecular hydrogen is used.
The other damaging phenomenon: creep, increases with increasing temperature within the reactor.
Reactors commonly used at present generally include an inner lining of austenitic stainless steel having a thickness of about 10 mm, and placed against the thick walls of ferritic steel. The main purpose of such lining is to protect the thicker walls from corrosion. It does not eliminate the damage for which hydrogen is responsible. Said lining does indeed retard surface decarburization of said walls to some extent, but it also has various unfortunate side-effects in that:
because hydrogen is very soluble in its structure (made of austenitic steel), it constitutes a source of hydrogen for the thick walls (made of ferritic steel) in contact therewith, and this source is present even after hydrogenation operations have being performed; and PA1 the interface between said lining and the thick walls against which it is placed constitutes a highly sensitive zone. Said interface constitutes not only a bridgehead for hydrogen which, once it has diffused through said lining continues to diffuse through said thick walls with the harmful consequences described above, but it also constitutes a trap for said hydrogen which finds room in defects of the interface to recombine and give rise to pockets of gas (molecular hydrogen, methane) under pressure. Said pockets of gas facilitate the detachment of the lining. PA1 said lining, which is made of austenitic steel, also has a low coefficient of diffusion for hydrogen and thus slows down desorption of hydrogen from the thick walls after hydrogenation operations have been completed. This requires a special very slow cooling procedure to be used in order to avoid the steel being made brittle by the hydrogen, which phenomenon shows up preferentially at the interface by said lining detachment. PA1 as a general rule, the walls are held together by welding, and said welds constitute bridges for the diffusion of atomic hydrogen; and PA1 the space created between the inner wall and said intermediate wall cannot withstand the severe operating conditions of hydrogenation reactors. PA1 balance pressure on either side of said inner wall (stabilize said inner wall), so that it does not have to withstand mechanical stress; and PA1 enable the hydrogen that has reached said space between the inner and outer walls to circulate and be exhausted. Said hydrogen is exhausted to the outside. In an advantageous use of a reactor of the invention, provision is made to keep the hydrogen pressure in said space as low as possible. In any event, provision is made to avoid creating in said space a hydrogenating environment of the same type as the reaction medium. PA1 the inner wall does not need to have strong mechanical characteristics; PA1 the outer wall is isolated from the "aggressive" hydrogen, and is indeed protected from hydrogen in general, so its thickness and its composition (content of carbon and various additives) can be optimized while ignoring said hydrogen and taking account only of creep; PA1 said outer wall can be raised to a temperature that is lower than the temperature of said inner wall (because of the interruption in heat transfer by conduction), with obvious consequences on its ability to withstand creep; and PA1 the above-mentioned interface problems are no longer encountered during cooling. PA1 an inner wall for coming into contact with the reaction medium and with a pressure-balancing fluid; PA1 optionally at least one additional wall for coming into contact on both faces with at least one pressure-balancing fluid; and PA1 an outer wall designed to be in contact with a pressure-balancing fluid and to withstand the pressure; PA1 an inner wall designed to come into contact both with the reaction medium and with a rigid structure; PA1 optionally at least one additional wall designed to come into contact on both faces with respective rigid structures; and PA1 an outer wall in contact with a rigid structure and designed to withstand pressure; PA1 firstly, the lifetime of the reactor can be extended by renewing the inner wall (only), should that be necessary; PA1 and secondly, a prior art reactor, possibly already in service, can be converted into a reactor of the invention specifically by inserting therein an inner wall in the meaning of the invention. PA1 at least one space between its inner and outer walls for controlled recombination of atomic hydrogen (said space also serving to interrupt heat transfer by conduction); said space containing no means liable to enable said atomic hydrogen to diffuse from said inner wall to said outer wall; and PA1 means for balancing pressures on either side of said inner wall and for enabling hydrogen to circulate in said space and to be exhausted therefrom. PA1 relatively pure argon is used in the empty space between the inner and outer walls, and is put into continuous or discontinuous circulation; or PA1 argon is put into continuous circulation in the first chamber of empty space between the inner and outer walls, which first chamber is defined by said inner wall and by an additional wall, and argon is renewed periodically (put into circulation discontinuously) in the second chamber of the empty space between said inner and outer walls, which second chamber is defined by said additional wall and said outer wall.
It should also be observed that, whenever the temperature varies, said interface suffers from stresses created by the differential thermal expansion of the respective steels constituting the lining and the thick walls. Said stresses also contribute to the lining detachment:
In such a context, at the present time, there is a permanent, ongoing search for a compromise in the choice of steel for use in the walls of hydrogenation reactors. It is indeed recommended to use steels of low carbon content in order to minimize reaction between the hydrogen and said carbon, however said steels must nevertheless contain sufficient carbon to have the required mechanical properties and to present sufficient resistance to creep. The carbon in said steels is stabilized by making use of carbides (e.g. of chromium, molybdenum, vanadium). In practice, given the temperature and hydrogen pressure parameters at which said reactors are used, appropriate steels are selected with the help of Nelson curves (API 941). These curves are familiar to the person skilled in the art and give temperature and hydrogen pressure resistance for various types of steel. The curves are established empirically.
Once said steels have been selected in this way (as a function of the temperatures and hydrogen pressures to which they are going to be subjected), reactors are always overdimensioned to take the presence of hydrogen into account, and the temperature at which they are used is limited for obvious safety reasons. As described above, said steels are also provided with an internal anti-corrosion lining.
At present, attempts are being made to develop new generations of steels enabling reactors to operate under even more severe conditions of temperature and hydrogen pressure, in order to obtain better reaction yields. All the research has been directed mainly to the behavior of steels in a hydrogenating environment (in the presence of absorbed hydrogen). (The behavior of the interface in the presence of hydrogen during temperature variations (thermocycling) is also a present topic of research.)
2. Description of Related Art
Patent GB-A-1 044 007 describes a reactor having an inner wall, an outer wall, and at least one intermediate wall which subdivides the space defined between said inner and outer walls into a plurality of compartments; said compartments contain a gas-permeable insulating material and they are kept in communication with each other and with the reaction medium. A flow of gas, e.g. CO2, is provided in said compartments from the outside and towards the reaction medium. Within such a structure, the outer wall is nevertheless not isolated from the reaction medium, and there are mechanical parts such as the bottom and the lid of the reactor which can constitute bridges for the diffusion of atomic hydrogen from the reaction medium into the outer wall.
Patent GB-A-2 135 901 describes another type of multi-walled reactor. To operate that type of reactor in a hydrogenating medium, a space is provided between the inner wall and the first intermediate wall, which space is open to the outside to exhaust the hydrogen to the atmosphere. It should nevertheless be observed that the proposed system does not prevent the hydrogen from attacking the adjacent walls and the outer wall, in that:
In those two types of prior art reactor, structure is optimized more with reference to the problem of thermal insulation than with reference to the problem of the walls being attacked by atomic hydrogen.